1. Field of the Invention
The present invention relates to a process for manufacturing a photomask and, more particularly, to process for manufacturing a photomask for use with manufacture of a semiconductor device.
2. Description of the Prior Art
FIGS. 1A to 1D show steps of a conventional photomask manufacturing process. First, as shown in FIG. 1A, a transparent glass substrate 1 is prepared and then a metal mask material 2 such as chromium is formed on the transparent glass substrate 1 by evaporation, sputtering or the like, with a thickness of 800-1000 .ANG.. Then, as shown in FIG. 1B, a resist 3 is applied onto the metal mask material 2, and a desired pattern is drawn by light or electron beam and then developing process is achieved, so that a resist pattern can be formed. Thereafter, as shown in FIG. 1C, exposed portion of the metal mask material 2 such as chromium is etched away by gas plasma process and the like and then the resist pattern is removed, so that a photomask called a hard mask, is formed as shown in FIG. 1D.
Previously, a mask used for manufacturing a semiconductor device included a photography emulsion dry plate using a transparent glass substrate. However, with advance of high integration and fining, a hard mask including a transparent glass substrate and a metal film such as chromium formed on the glass substrate has been widely used. More particularly, in case of a hard mask such as chromium, a thinner film of chromium can be used, as compared with a conventional emulsion mask and hence it becomes possible to obtain a finer pattern and lifetime of mask becomes longer.
However, at the same time, an etching technique of a metal mask material 2 of a hard mask such as chromium is important. In a case where a metal film is made of chromium, a wet chemical etching process is generally used, in which a mixture solution of ammonium cerium (IV) nitrate and perchloric acid is employed. On the other hand, with advance of technique for fining a pattern, a dry etching technique has been developed and utilized, in which a gas plasma or reactive ion etching is employed. In case of a plasma etching of chromium, a chromium is etched by grow-discharging a mixed gas containing a halogen element such as chlorine and a oxygen, with the reaction being considered as follows; EQU Cr+20+2Cl.fwdarw.CrO.sub.2 Cl.sub.2
As described in the foregoing, a hard mask of chromium and the like is advantageous for forming a fine pattern, while there is a disadvantage that a rate of etching, particularly a dry etching using a gas plasma, is low. More particularly, in case of chromium, the etching rate is about 100 .ANG./min in a condition of 300 W and 0.2 Torr and 8-10 minutes of etching time is required (in case of 800-1000 .ANG. in thickness). In addition, reduction of thickness of resist film due to a longer time required for etching is also a problem.
Incidentally, Japanese patent application No. 42176/1981, filed Mar. 23, 1981 and laid open for public inspection Sept. 28, 1982 and Japanese patent application No. 42183/1981, filed Mar. 23, 1981 and laid open for public inspection Sept. 28, 1982 disclose that a silicon layer and a metal layer are deposited on a glass substrate so that a pattern of silicide is formed by an electron beam.